1. Field of the Invention
The present invention relates to a vehicle air conditioning system that controls compressor cycling in order to reduce engine workload. More specifically, the present invention relates to a vehicle air conditioning system that controls compressor cycling in response to estimated humidity conditions at the air conditioning system evaporator.
2. Background Information
Vehicle air conditioning systems typically maintain the temperature of the passenger compartment by operating a compressor to keep the evaporator at a temperature just above the freezing point of water. Air passing through the evaporator is cooled but is typically too cold to send directly into the passenger compartment. In recent model vehicles, a portion of the cooled air is re-heated by passing the portion of the cooled air through the heater core. The cold air and the re-heated air are mixed together prior to entering the passenger compartment in order to deliver cooled air that is not frigid.
This conventional way of operating the vehicle air conditioning system is inherently inefficient. Energy is wasted by cooling the evaporator to a temperature just above the freezing point of water regardless of the cooling requirements. Further, the re-heating of cooled air represents an additional waste of energy. Further, since the evaporator is cooled to a temperature that is close to, but above the freezing point of water regardless of cooling requirements of the air conditioning system, the cooled air is substantially dehumidified. Specifically, the majority of the moisture in the air passing through the evaporator condenses on the outer cooling surfaces of the evaporator. This moisture then typically drains out of the evaporator casing and falls beneath the vehicle. This results in an over-drying of the air entering the passenger compartment. For the comfort of the passengers, it is not necessary to remove all of the moisture from the cooled air.
From a driver visibility perspective, it is only necessary to remove sufficient amounts of moisture from the cooled air to prevent fogging on the interior surfaces of the windows of the vehicle. From a comfort perspective, it is only necessary to remove sufficient amounts of moisture to make passengers feel cool and comfortable